Machine-readable symbols (MRSs) provide a means for encoding information in a compact printed form (or embossed form) which can be scanned and then interpreted by an optical-based symbol detector. Such machine-readable symbols are often attached to (or impressed upon) product packaging, food products, general consumer items, machine parts, equipment, and other manufactured items for purposes of machine-based identification and tracking.
One exemplary type of machine-readable symbol is a bar code that employs a series of bars and white spaces vertically oriented along a single row. Groups of bars and spaces correspond to a codeword. The codeword is associated with an alpha-numeric symbol, one or more numeric digits, or other symbol functionality.
To facilitate encoding of greater amounts of information into a single machine-readable symbol, two-dimensional (2D) bar codes have been devised. These are also commonly referred to as stacked, matrix and/or area bar codes. 2D matrix symbologies employ arrangements of regular polygon-shaped cells (also called elements or modules), typically squares. The specific arrangement of the cells in 2D matrix symbologies represents data characters and/or symbology functions.
In this document the terms “barcode” and “symbol” are employed interchangeably, both generally referring to machine-readable symbols, whether linear or two-dimensional.
Symbol readers (or barcode readers), also referred to as scanners, are employed to read the matrix symbols using a variety of optical scanning electronics and methods. In order to properly scan a symbol, the symbol must be within a field of view of a reader. Some readers are hand-held, and can be aimed at a symbol; other readers are fixed in location, and a symbol (and the object to which the symbol is attached) must be placed within a field of view of the reader.
Either way, a symbol scanner may project an “aimer pattern” or “aimer beam”—a pattern of light—which may indicate the scanner's center of the field-of-view; the aimer pattern may also project/include corner patterns to indicate the edges of the field of view. Proper alignment or overlap of the projected aimer pattern with the target symbol indicates that the scanner is properly aimed for scanning.
Once the scanner and symbol are properly aligned to have the symbol in the field of view of the scanner, the scanner proceeds with image capture via an imaging element. The aimer beam typically must be turned off when the imaging element captures a symbol image, because the aimer pattern is visible to the imager and becomes noise superimposed on the symbol. Existing aimers, even if centered on a specific color (red, amber, green), all have high intensity in wavelengths to which the image sensors are sensitive. As a result, aimer illumination of a symbol is easily captured, which can disrupt symbol interpretation. In other words, the imager cannot reliably capture the symbol's image when aimer is on. This is true for most commonly used image sensors with electronic rolling shutters. Consequently, the time spent with the aimer being “on” (illuminating the symbol) directly reduces the imager barcode reader response speed.
In the alternative, for global shutter image sensors, reduced response speed may be less of a problem, as the aimer can be on during the shutter-closed portion of the whole image capture cycle. But for the global shutter image sensors, the short on-period aimer also becomes less visible because of the limitation of the aimer light source output power. An on-and-off aimer also introduces a flashing pattern which induces eye fatigue in users.
One approach to resolving this problem is to use a constant-on aimer with less contribution to the overall image illumination; for example the aimer pattern may be thin or have dotted line patterns. However, for 2D symbols with high density, poor print quality, or with 2D codes with lower redundancy rates, this trade-off will introduce poor decode rate.
Therefore, there exists a need for a system and method for both aimer illumination and symbol capture illumination which avoids time-sharing between the aiming process and symbol capture, yet still achieves a high level of accurate performance in symbol decoding.